Earthing, Bonding, and Protection Against Electric Shock
Earthing, bonding and protection against electric shock is where the most marks are lost, because the answer depends on the declared earthing system (TN-S, TN-C-S, TT, IT) and on whether faults are cleared by overcurrent devices or by residual current devices.
Core rules
- Exposed conductive parts are connected to the protective conductor (PE); extraneous conductive parts are connected by main protective bonding to the main earthing terminal. Water, gas, structural steel and lightning protection all terminate at the bonding bar.
- Automatic disconnection requires Zs ≤ U0/Ia, where Ia is the current that operates the device within the stated time (5×In for a B MCB, 10×In for a C MCB, IΔn for an RCD).
- The touch-voltage limit is 50 V AC under normal conditions.
- In TN-C-S the PEN is earthed at the origin and split into PE and N; downstream, PE and N must never be recombined.
- In TT the installation has its own electrode and RA ≤ 50/IΔn.
Worked example
A 16 A B-curve MCB protects a socket circuit. Ia = 5 × 16 = 80 A, so maximum Zs = 230/80 = 2.88 Ω. A measured Zs of 1.9 Ω confirms disconnection. If the measurement were 3.4 Ω the circuit fails; you must improve the earth path or add a 30 mA RCD, for which RA ≤ 50/0.03 = 1667 Ω.
Traps
- Assuming an RCD removes the need for a sound earth connection.
- Losing the PEN and diagnosing it as a simple neutral fault.
- Using a metallic water pipe as the only earth electrode.
- Omitting supplementary bonding in bathrooms and around metallic services.